In general, silicone resins have high gas permeability. Therefore, it was previously difficult to use silicone resins for general applications requiring gas resistance. Japanese Patent No. 4114037 describes a method where metallic powder is added in a silicone rubber composition to prevent a sulfur-containing gas from reaching electric and electronic components. However, transparency of the silicone rubber composition is poor. Japanese Patent No. 4009067 describes an addition-curable phenyl silicone resin composition which is transparent and highly hard. However, gas permeability of the resin composition is not low.
Further, silicone resin compositions are widely used as optical semiconductor encapsulating materials. However, in recent years, high brightness LEDs which generate high intensity light and a large amount of heat has been commercialized. Therefore, discoloration of an encapsulating material caused by a corrosive gas has been a problem. Japanese Patent Application Laid-Open No. 2005-272697 discloses that a hinderdamine light degradation-preventive agent is added to a phenyl silicone resin to provide an encapsulating material having good heat resistance, stable light resistance and good weather resistance. Japanese Patent Application Laid-Open No. 2009-215434 discloses that a silicone resin having an aliphatic hydrocarbon group as a substituent, such as a methyl silicone resin, prevents degradation of an organic resin package to extend a life of LED.
The afore-mentioned silicone resins have good light resistance, heat discoloration resistance and impact resistance, but have large linear expansion coefficient and high gas permeability and, therefore, have a problem in a reliability of an encapsulating material, which problem is caused by a corrosive gas. Further, it is known that the high gas permeability of the silicone resin leads to erosion of a silver plated surface in a substrate which is used in an optical semiconductor device such as an LED and, eventually, to decreased brightness. Therefore, there is a need to improve the silicone resin.